Passive components in the back end of integrated circuits

ABSTRACT

Passive components are formed in the back end by using the same deposition process and materials as in the rest of the back end. Resistors are formed by connecting in series individual structures on the nth, (n+1)th, etc levels of the back end. Capacitors are formed by constructing a set of vertical capacitor plates from a plurality of levels in the back end, the plates being formed by connecting electrodes on two or more levels of the back end by vertical connection members.

TECHNICAL FIELD

The field of the invention is that of forming passive components in theback end of the line wiring of an integrated circuit.

BACKGROUND OF THE INVENTION

Conventionally, the formation of passive components, e.g. resistors andcapacitors, in the back end employs one or more photolithographic masksto define the deposition of a high-resistance material in the case ofresistors and the deposition of a pair of planar electrodes with adielectric between in the case of a capacitor.

In the case of resistors, the material has a high resistivity comparedwith the Al or Cu of the interconnect material and therefore requires aseparate deposition step and a special mask different from the mask ofthe nth wiring level. In the case of capacitors, there will be separatedeposition steps for horizontal planar electrodes and for the dielectricbetween the electrodes.

Structures like these perform well enough, but the requirement of one ormore additional masks means that the use of such structures incursadditional costs to have the masks made, plus yield detractorsassociated with the additional handling and processing steps.

The art could benefit from a method of forming passive components thatdoes not require additional masks and/or processing steps.

SUMMARY OF THE INVENTION

The invention relates to a method of forming passive components in theback end that does not require the use of an additional mask.

A feature of the invention is that the structure is formed by combiningwires and vias defined by the same masks used for interconnections inthe back end.

Another feature of the invention is that structures on different levelsof interconnection are combined so as to reduce variability in the valueof the resistance or capacitance of the device being formed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an exploded view of a first embodiment of a resistor.

FIG. 2 shows a top view of an alternative embodiment of a resistor.

FIG. 3 shows a side view of an alternative embodiment of a resistor.

FIG. 4 shows another side view of an alternative embodiment of aresistor.

FIG. 5 shows a top view of an embodiment of a capacitor.

FIG. 6 shows a cross section of the capacitor of FIG. 5.

FIG. 7 shows another cross section along a perpendicular direction.

FIG. 8 shows a graph indicating the relationship between variability andthe number of levels.

FIGS. 9 a and 9B show top views of an alternative embodiment

DETAILED DESCRIPTION

Passive Resistors:

FIG. 1 shows in an exploded view a portion of a resistor formedaccording to the invention, in which a first resistive element 10 formedon the nth metal level and a second resistive element 15 formed on the(n+1)th metal level are connected in series through one or more vias,e.g. two vias, v1 filled with a conductor and connecting contacts 13-non the nth level and a corresponding contact on the (n+1)th level,13-(n+1). The two vias are denoted with dashed lines. Furtherconnections are provided at contacts 12-n, connecting to resistiveelements in layers below those shown and contact 12-(n+1) connecting toresistive elements in layers above those shown.

Box 3 on the right represents schematically other interconnections onthe (n+1)th level in other parts of the circuit being constructed. Box 2also represents schematically other interconnections on the nth level inother parts of the circuit being constructed.

Each resistive element in FIG. 1 is formed in a serpentine shape fromthe same material, e.g. Cu or AlCu lined with thin refractory metal orrefractory metal alloys, as known in the art, as is used to form theinterconnects on that wiring level. That is, the same mask is used topattern simultaneously the interlevel dielectric (e.g. to form adamascene structure and the damascene apertures for the resistiveelement) and corresponding structures for the interconnect.Alternatively, if subtractive-etch wiring, e.g. AlCu wiring, is used,then the same mask is used to pattern the AlCu wires and the resistiveelement wires.

The example in FIG. 1 shows only two levels for convenience inillustration, but any desired number of levels can be used to form thepassive elements.

The designer of the product containing the structure (e.g. an integratedcircuit) will decide what will be the cross section of the resistiveelement. In a preferred embodiment, a cross section for the wire in theresistive element is greater than 1× (e.g. 1.4) and in a range ofgreater than 1× to 2× the corresponding minimum cross section width ofthe regular interconnect. This counterintuitive result, since theresistance will thereby be lowered, results from a desire to reducevariability of the total resistance of the final structure. The wireresistance variability is determined by the variation in width, height,and conductor resistivity. For minimum width wires, the variability istypically dominated by the wire width variation. Thus, for example, a200 nm wide wire will have 40% less resistance variability than a 100 nmwide wire. In addition, more than one via is preferably used in aparallel connection as shown in FIG. 1 to connect layers (resistiveelements) of the structure. Duplication of vias will reduce the totalresistance, but will also reduce via resistance variability and increasereliability.

FIG. 2 shows a serpentine 10 as in FIG. 1, with a set of protectivebuffer structures 17 and 18. These structures are damascene trenchescontaining the same metal as the serpentine 10. The function of thebuffer structures is to reduce wire width and height variability inducedby lithography, etching, and chemical-mechanical polishing (CMP) of theserpentine. During photolithography, due to light scattering effects,the wires at the edge of an array tend to print larger or smaller thanwires in the array and thus have more width variability. During etching,due to RIE loading effects, wires at the edge of nested arrays haveincreased width and, for damascene wires, height variability. DuringCMP, wires at the edge of nested arrays have increased heightvariability, due to surface erosion variability induced by thedielectric adjacent to the wires. Thus, the buffer structures 17 and 18help to protect the edges of serpentine 10, thereby reducing resistancevariability.

Table I shows measured wire reistance variability (the range from 5^(th)to 95^(th) percentile, measured on both 200 mm and 300 mm wafers withapproximately 20 sites per wafer measured) for various wire serpentinestructures. For the 130 nm generation fluorinated SiO₂ (FSG) dielectric,the measured wire resistance variability decreased substantially as thewire width increased from 200 nm to 3 um. For all structures, wiringgenerations, and dielectrics, the measured wire variability decreased asthe number of wiring levels used increased from 1 to 2 to 3.

TABLE I Variability of Resistance vs. the Number of Levels Technology 1level 2 levels 3 levels 200 mm wafer 130 nm 22.2% 19.7 17.6 generationFSG dielectric w = 200 nm 200 mm wafer 130 nm 17.4 13.0 13.0 FSG w = 3micron 300 mm wafer 90 nm 24.5 22.4 20.4 FSG w = 140 nm 300 mm wafer 90nm 25.8 22.0 22.0 SICOH w - 140 nm

FIG. 3 shows a cross section of an alternative embodiment of a resistor.A connection is made from the bottom through via 31 to a segment(resistive element) of metal 1, labeled 31-1. Illustratively, the lengthof metal 31-1 (and other resistive elements) is below the criticallength required for Cu or AlCu electromigration to occur (e.g. on theorder of 1-20 microns) to take advantage of the short length effect thatreduces the effect of electromigration in metal under electrical stress.Similar resistive elements on level 2, 32-2, level 3, 32-3, and level 4,32-4, are connected. Duplicate vias 31-1, 31-2 and 31-3 provide reducedvariability as in the previous embodiment. The segments are shown asstraight lines, but may be a serpentine as in FIGS. 1 and 2 or any otherdesired shape.

FIG. 4 shows a folded version of the embodiment of FIG. 3, with thelevel 3 resistive elements being located above the level 2 elements.This embodiment provides a more compact arrangement, with savings inarea; and reduces the resistance variability because multiple wiringlevels are used.

Passive Capacitors:

On chip capacitors can be formed using the self or natural capacitanceof the BEOL wires (J. Kim et al., IEEE 2003 Symp. on VLSI Circuits, p29). By employing methods similar to those discussed supra forresistors, the vertical natural capacitor (VNC) capacitance can beincreased, the capacitance variability can be decreased, and thereliability, as measured by ramping the voltage until the capacitorfails, performing time dependent dielectric breakdown stressing (TDDB),or calculating the yield to a specific leakage current criteria, can beimproved.

One method of increasing the capacitance density of VNC capacitors is toinclude vias between the interpenetrating comb structures. FIG. 5 showsa top view of an embodiment of a capacitor 50 according to theinvention, in which the capacitor structure consists of twointerdigitated electrodes 55 and 52, illustratively formed by adamascene process in an interlevel dielectric. Arrows 6-6 and 7-7 denotethe locations of cross sections shown in FIG. 6 and FIG. 7,respectively. The capacitor comprises a set of several levels ofelectrodes tied together with vertical connectors 53 placed in vias. Inthis example, the vias for the two electrodes are aligned with oneanother, i.e. both electrode 55 and electrode 52 have vias in adjacentrows k and k+1. The space in FIG. 5 between electrodes 55 and 52 isillustratively the minimum permitted by the ground rules. In addition,the width of the conductive material in the finger electrodes is greaterthan the minimum width, in order to reduce variability and permit thefabrication of borders on the vias. The borders are denoted with bracket54 in FIG. 6. These borders, fabricated conventionally, serve to reducethe risk of shorts between vias. For example, if the minimum wire widthand space is 100 nm, then a 100 nm space between wires could be used anda 140 nm wire width could be used, This would result in a 20 nm per edgeborder on the vias, which would reduce the chance of a misaligned via oroverwidth via shorting to the adjacent wire or via.

FIG. 6 shows a cross section along line 6-6 in FIG. 5, with five levelsof electrodes 55 and 52 tied together with connectors 53. Both thedamascene electrodes and the vertical connectors will contribute to thecapacitance of the structure.

FIG. 7 shows a cross section through line 7-7, showing a portion of thestructure. The result is an effective vertical capacitor plate formedfrom the finger electrodes of the electrodes and the vias. It can beseen that in this example, the vias will contribute to the capacitancein this version, since they are aligned (as shown in FIG. 5) oppositeone another.

Table II shows the effect of decreasing the transverse dimension of viasin a structure such as that shown in FIG. 6, in which solid fingerelectrodes of metal, deposited with the same mask as other interconnectstructures on that metal level, are connected vertically by metalconnections with spaces between the vertical connectors, so that thevertical structure is not solid. Adding minimum spaced (dense) viasincreases the capacitance density by 20%, as expected due to increasedelectric field coupling between the VNC plates due to the addition ofvias. To improve the yield and reliability of this VNC structure, it isdesirable to decrease the via density by staggering the vias, as shownin FIG. 9A (top view). In this case, the via spacing is increased from1.0× the via width to 3.0× the via width; and the vias are centered tomaximize the space between vias on different plates of the VNCcapacitor. As shown in Table II, reducing the via density by staggeringthem only decreases the capacitance density by ˜1%, because a highenough via density is still present.

TABLE II Structure Normalized Capacitance Density No vias, minimum pitch1 Bordered Dense Vias 1.20 (larger pitch) Bordered staggered 1.19 densevias

FIG. 8 shows the effect on variability, e,g, capacitance, of increasingnumber of metal levels, for various dimensions and intermetal dielectricmaterials complementing the data in Table I. The vertical axis is theelectrically measured range between the 5th and 95th percentile of theparameter (capacitance) plotted against the number of metal levels It isevident that the variability decreases as the number of levels of metalincreases.

FIGS. 9A and 9B show detailed top views of the staggered—via VNCembodiment, in which the vias are staggered; i.e. the vias connectingthe layers of electrode 52 are located on rows N and N+2, etc. while thevias connecting the layers of electrode 55 are located in between rows Nand N+2, i.e. on row N+1, etc. This layout slightly reduces thecontribution to the capacitance from the vias, but also reduces the riskof short circuits caused by misalignment of the connectors 53 relativeto the solid fingers 52 and 55. For example, with 100 nm spaces betweenthe wires and 140 nm wide wires, staggering the 100 nm wide vias with noreduction in via density decreases the capacitance by approximately 0.2%(FIG. 9 a). If staggered vias are used with a 50% reduction in viadensity (i.e. the via spacing is increased from 100 nm to 300 nm), thenthe capacitance is decreased by approximately 1% (FIG. 9 b). In bothcases, the potential for via-induced shorting between wires issignificantly reduced with a negligible decrease in capacitance.

FIG. 9A shows a top view of a capacitor similar to that of FIG. 6, buthaving staggered vertical electrodes connecting the different levels ofthe fingers 55-1 through 55-6 of electrode 55 and 52-1 through 52-5 ofelectrode 52. Brackets 92 in FIG. 9A denote a basic cell extending alonga finger and having two vertical connectors in finger 55-1 and also twovertical connectors in finger 52-1 of the other electrode. Brackets 94in FIG. 9B denote corresponding 2× cells having vertical connectorsspaced by twice the amount as those in FIG. 9A. vertical connectorswires are spaced by a greater amount because the height of theconnectors is increased. Illustratively, the fingers are 180 nm wide andthe vertical connectors have borders of 20 nm.

In general, the wiring pitch (pitch=minimum wire width+minimum wirespace) is not constant for multilevel wiring. For the 65 nm generation,illustrative wiring pitches are given below, and the wire width andspace are approximately ½ the pitch:

M1—160 nm

M2-M6—200 nm

M7-M8—400 nm

M9-M10—800 nm

When the wires are used to form resistors or capacitors, it is desirableto have a repeating unit cell. One method of achieving this is to usethe M2-M6 wiring as a template, with the M1 wires having samewidth/space as M2; M7-M8 having double the pitch, and M9-M10 havingquadruple the pitch: An example is shown in the following Table III

TABLE III Wire Wire Wire Via Via border level Min. wire width widthspace width per edge M1  80 nm 140 nm 100 nm 100 nm 20 nm M2-M6 100 nm140 nm 100 nm 100 nm 20 nm M7-M8 200 nm 280 nm 200 nm 200 nm 40 nmM9-M10 400 nm 560 nm 400 nm 400 nm 80 nm

Although, in this example, a simple wire pitch doubling algorithm isused, any algorithm which results in a repeating unit cell could beused. FIGS. 9A and 9B show one example, with FIG. 9A showing thetemplate level (M2 in the table above) and FIG. 9B showing a 2× spacing(M7 in the table above).

As with any wiring structures, it is desirable to minimize wire to wireor via to wire/via shorting both during processing and while the chip isin the field. For large minimum space structures, such as the capacitorsor resistors discussed in this disclosure, a reaction between thedielectric and non-inert atmosphere (e.g. humid air) can occur,resulting in either the growth of Cu containing particles between wiresor the degradation of the electrical insulative properties of theintermetal dielectric due to water absorption. To prevent this ambientatmosphere-induced degradation, particularly for FSG and carbon-basedoxide (SiCOH) intermetal dielectrics (IMD), the time between Cu CMP andthe post CMP dielectric cap deposition should be minimized and keptbelow a time window threshold amount. In particular, for FSG-IMD, thetime window should be less than 16 hours, e.g. 6 hours; and, forSiCOH-IMD, the time window should be less than 120 hours, e.g. 24 hours.This allows for flexibility in manufacturing without degradation of theVNC yield or reliability, as determined by voltage ramp to fail or TDDBstressing. For films sensitive to water absorption, a bake-out, e.g.400C in a vacuum or inert ambient for 1 hour, rework step could beemployed for wafers which exceed the time window.

The following claims refer to the first and second wiring levels. Itwill be evident to those skilled in the art that the first wiring levelof a structure may be on the second, third, etc metal wiring level ofthe back end.

The disclosure has used a dual damascene technique for illustration, asthis technique is economical. The invention can also be practiced withseparate steps for the interconnections and for the vias

While the invention has been described in terms of a single preferredembodiment, those skilled in the art will recognize that the inventioncan be practiced in various versions within the spirit and scope of thefollowing claims.

1. A method comprising: forming at least two wiring levels containinginterconnect members upon a substrate; forming a passive circuit elementin said at least two wiring levels having a first portion formed on afirst wiring level simultaneously with interconnect members formed onsaid first wiring level and a second portion formed on a second wiringlevel simultaneously with interconnect members formed on said secondwiring level, further comprising: forming a capacitor comprising atleast two sets of interdigitated electrodes in each of said first andsecond wiring levels, forming each of said interdigitated electrodesconnected vertically to corresponding electrodes in the other of saidfirst and second wiring levels by a set of vertical connection members,thereby forming an effective vertical plate, in which said verticalconnection members are arrayed in rows disposed along finger electrodesof each of said interdigitated electrodes, with vertical connectionmembers connecting said finger electrodes located in each of said firstand second wiring levels, in which said vertical connection members aredisposed in staggered rows disposed along said finger electrodes, with arow of vertical connection members connecting finger electrodes of oneof said interdigitated electrodes alternating with a row of verticalconnection members connecting finger electrodes of the other of saidinterdigitated electrodes; forming said finger electrodes having afinger width greater than the width of said vertical connection members;and forming in at least one of said first and second wiring levelsvertical connection members arrayed in a set of unit cells having afirst pitch, and forming in at least one other of said first and secondwiring levels vertical connection members arrayed in a set of unit cellshaving a second pitch greater than said first pitch.
 2. A methodaccording to claim 1, in which said second pitch is twice said firstpitch.
 3. A method comprising: forming at least two wiring levelscontaining interconnect members upon a substrate; forming a passivecircuit element in said at least two wiring levels having a firstportion formed on a first wiring level simultaneously with interconnectmembers formed on said first wiring level and a second portion formed ona second wiring level simultaneously with interconnect members formed onsaid second wiring level, further comprising: forming a capacitorcomprising at least two sets of interdigitated electrodes in each ofsaid first and second wiring levels, forming each of said interdigitatedelectrodes connected vertically to corresponding electrodes in the otherof said first and second wiring levels by a set of vertical connectionmembers, thereby forming an effective vertical plate, in which saidvertical connection members are arrayed in rows disposed along fingerelectrodes of each of said interdigitated electrodes, with verticalconnection members connecting said finger electrodes located in each ofsaid first and second wiring levels, wherein said vertical connectionmembers are disposed in staggered rows which are perpendicular to saidrows disposed along said finger electrodes, with a staggered row ofvertical connection members connecting finger electrodes of one of saidinterdigitated electrodes alternating with a staggered row of verticalconnection members connecting finger electrodes of the other of saidinterdigitated electrodes; and, forming said finger electrodes have afinger width greater than the width of said vertical connection members.4. A method according to claim 3, in which said vertical connectionmembers and said finger electrodes are formed in a dual damasceneprocess.
 5. A method according to claim 3, in which said verticalconnection members have a border, thereby having a width greater than aminimum linewidth and less than or equal to said finger width minustwice the magnitude of said border.